Silicon integrated circuit chips are typically packaged in sealed enclosures, such as chip carriers or dual in-line packages (DIP), and interconnected with other components on printed circuit boards. The integrated circuit is typically connected to a carrier substrate with the active or upper surface of the integrated circuit facing away from the carrier substrate. Interconnections between the IC and the substrate is typically made by thin metal wires or wirebonds welded between the pads of the IC and the circuitry on the printed circuit board.
In an alternate packaging configuration known as flip chip, the integrated circuit is bonded directly to the substrate by means of solder bumps or other metal interconnects and is mounted with the active surface facing the printed circuit board. This type of interconnection eliminates the expense, unreliability, and low productivity of manual or automated wirebonding. It also allows the use of interior or array contacts as opposed to the limitation of using peripheral contacts with wirebonding methodology. The flip chip or control-collapse-chip-connection (C4) utilizes solder bumps deposited on wettable metal terminals of the chip and a matching footprint of solder wettable terminals on the substrate. The upside down flip chip is aligned to the substrate, and all interconnections are made simultaneously by reflowing the solder. This is opposite to the traditional method of bonding where the active side of the chip is facing up and is wirebonded to the substrate.
When chips are mounted in the conventional manner, that is, wirebonded, a ground connection to the backside ground plane of the chip can be made by forming a conductive connection between the backside ground plane of the chip and the ground of the printed circuit board. This is easily performed by means of conductive epoxy or solder, for example. In flip-chip devices the connection of the ground plane of the IC to the ground of the substrate is more difficult because now the ground plane is facing away from the substrate. Wirebonds may be formed between the ground of the IC and the ground of the substrate but this negates the advantages realized by the flip-chip mounting scheme.
A method of achieving a ground connection between the back side ground of the flip chip and the ground of the circuitry is needed that eliminates the cumbersome, costly and unreliable wirebonds.